1. Field of the Invention
The present invention relates to the field of optical instruments, and in particular to reflecting telescopes having an ultra-wide field of view, above 6°×6°.
Telescopes with ultra-wide field of view are designed for applications such as imaging in the visible and in the infrared spectral domain, spectral imaging, earth and planetary observation, and detection of fast moving faint objects.
2. Description of the Prior Art
One difficulty with all-reflective systems is to achieve good image quality over a wide field of view while minimizing the sizes of individual mirrors and the volume of the telescope package containing the mirror elements. A single mirror with an appropriately shaped surface is capable of forming a perfect geometric image of a single object point. The mathematical description of the surface of the mirror is dictated by the fundamental requirement that the length of any path from the object to the image be equal to the length of any other similar path. For an infinitely distant object point, for example, the theoretical figure which achieves perfect geometric imagery is the paraboloid of revolution. The ideal mirror shape produces the best possible geometric image quality for a given pair of object and image conjugate points. Errors in fabrication distort the actual mirror surface from the ideal shape and cause the size of the geometric image to grow, degrading the frequency content of the image. Even in the absence of errors, diffraction resulting from the finite size of the collecting mirror aperture places an ultimate lower diffraction limit on the size of the image spot for a given system. When the geometric image spot size from the ideal mirror shape with fabrication errors is smaller than the spot size caused by diffraction, the system is said to be diffraction limited. Optimum telescope systems operate near the diffraction limit to provide the highest resolution possible with the least degradation of the frequency content.
An extended object can be considered as a continuum of object points each of which is subject to distortion from perfect imaging by diffraction and optical aberrations. A single mirror surface is generally not capable of perfect imaging for more than one object point and image point. Except in special, impractical cases, a single mirror cannot form a perfect image of extended objects. Hence, optical systems must add additional reflective surfaces to provide near perfect imaging of extended objects. Additional surfaces provide additional degrees of freedom which define the shapes and locations of mirror surfaces. Thus, a multiple mirror system has a set of surfaces and spacings defining path lengths traversed by rays propagating from the object to the image. The path lengths for each point are equal for enough object points to span the required angular field of view. There is a design margin that does not significantly affect image quality because geometric spots need only be smaller than the diffraction limit. Design difficulty increases with an increase in the field of view of the object to be imaged because of the difficulty in maintaining equal path lengths for an increasing number of conjugate points. Additional mirror surfaces and more complex mirror surface shapes such as aspheric surfaces have been used to meet the demands of high quality imaging of large extended objects, or a large field of view for objects with infinite object distance. The manufacturing criticality in general increases if additional constraints are required for the optical system like flat image plane, low distortion and/or telecentricity.
Aspheric surfaces, which cannot be represented as part of a large sphere, include conic sections such as hyperboloids, paraboloids, ellipsoids and oblate spheroids. A conic section is one of several possible shapes derived from an intersection of a plane with a cone. There are also general aspherics, for which the shape of the surface is represented by a general polynomial equation in one of several established forms. The use of aspheric surfaces is well known. For example, ellipsoidal and hyperboloidal mirrors are described in U.S. Pat. No. 4,101,195. Both mild and strong aspherics have been used and are characterized by the extent of departure from spherical.
Aspheric surfaces are more difficult to manufacture than spherical surfaces and are more complicated to design as part of an optical system. Spherical mirrors are always rotationally symmetric because any section of a spherical mirror is the same as any other section. Aspheric surfaces can, however, replace several spherical surfaces for the purpose of reducing aberrations.
One prior art telescope with a wide linear field of view is shown and described in U.S. Pat. No. 4,240,707. In this prior art three-mirror-anastigmatic type telescope, high resolution imaging is achieved with an aperture stop centered on a convex secondary mirror and with concave primary and tertiary mirrors eccentric sections of larger symmetric parent reflective surfaces. The surfaces are typically aspheric, including for example, conic sections, depending on the field of view and image quality desired. With conic aspherics, linear fields of view of up to five degrees on a flat focal surface have been demonstrated. With general aspherics, linear fields up to 15 degrees have been demonstrated. The reflective triplet operates most effectively at focal lengths from three to six times the aperture size; that is, with focal ratios of f/3 to f/6. The focal ratio, often called the f-number, is the ratio of the focal length divided by the aperture diameter.
Four mirror systems are also known, for example, the one disclosed in U.S. Pat. No. 5,142,417. This document describes an f/12 to f/20 optical system with a large effective focal length. This system comprises spherical mirrors which are advantageously easy to manufacture, but disadvantageously have disjointed mirror axes resulting in a complex alignment of the telescope, in a difficult straylight baffling and in a relatively small angular field of view. The disjointed mirror axes also particularly increase system complexity and alignment requirements, presenting difficult manufacturing challenges.
U.S. Pat. No. 5,640,283 also discloses a four mirror system comprising two concave primary and tertiary mirror surfaces and two convex secondary and quaternary mirror surfaces. The tertiary and quaternary mirrors are spherical. The secondary mirror is a mild circular convex ellipsoid. This telescope is limited to a linear field of view of 5.6° and has a focal surface which is not flat but in the form of a concave cylinder section.
U.S. Pat. No. 6,767,103 also describes a four-mirror telescope with three aspheric mirrors and a spherical field mirror with a sequence concave, convex, convex, concave. This instrument is designed for high resolution imaging of a distant object, and has a narrow field of view of 0.4°×0.4°.
There is a need for a telescope having an ultra-wide field of view combined with a high imaging performance, a flat image focal plane, and a minimum distortion in all the field of view (image space), which is in addition telecentric in image space. This means that the chief rays in image space for all field directions are parallel (the exit pupil of the system is located at infinity).